AC motor system

ABSTRACT

An alternating current (AC) motor system includes an AC motor having a rotor and a stator with the rotor having a plurality of air passes for passing cooling air and the stator having end windings extending from each end. An air deflector is coupled to one end of the rotor and has a generally curved configuration for directing cooling air exiting the air passages into a generally radial flow direction and onto the stator end windings. The stator end windings comprise turns of a plurality of electrical conductors forming the stator winding circuits which are wound to form a plurality of phase windings. Controllable switches are connected to the winding circuits for coupling the phase windings into either a parallel or series circuit arrangement. The system is responsive to motor speed and operating state for selectively switching the windings between the series and parallel electrical circuit arrangements. Resistors are coupled in series electrical circuit with each of the electrical phase windings and controllable switches are coupled in parallel with the resistors for selectively bypassing phase current around the resistors.

The present invention relates to electric traction motor vehicles and,more particularly, to an alternating current (AC) motor and drive systemfor such vehicles.

BACKGROUND OF THE INVENTION

Alternating current motors and in particular the squirrel cage inductionmotor are generally known to be more rugged than an equivalent sizedirect current motor. The development of high power semiconductors hasmade the use of such motors in variable speed traction drives practical.In such applications, the torque developed by AC induction motors is afunction of the voltage applied to the primary or stator windings andmotor speed is a function of the frequency of the applied voltage. Themaximum torque capability of the motor (known as "Break Down Torque", orBDT) varies directly with motor flux squared. The motor's flux variesdirectly with applied voltage and inversely with frequency (speed).Therefore, when such a motor is operating at constant voltage and speedis increased, its maximum torque producing capability is decreasing withthe square of the speed increase. Thus, even when operating on aconstant power output characteristic (i.e. torque demand decreaseslinearly with speed increase) a speed could eventually be reached wherethe power level which could be maintained at lower speeds cannot be heldat higher speeds due to the loss of torque producing capability of themotor. Solid-state, i.e., semiconductor, inverters can provide suchvariable frequency, variable voltage power for the AC motor. The voltageapplied may be increased up to either the maximum voltage rating of theinverter or of the motor in order to maintain a desired torque as themotor accelerates. The motor and inverter are selected to provide apredetermined torque at start-up and for operation at low speed. At somehigher speed, voltage limits are reached and voltage is generally heldconstant allowing torque to decrease as speed increases.

As a practical matter, it is not desirable to operate a motor at or verynear its breakdown torque since such breakdown torque varies with thesquare of the flux density and flux density varies directly with voltageand inversely with frequency. In general, motor operation is limited tosome preselected torque margin, such as, for example, 1.3 or higher,where torque margin is defined as the ratio of breakdown torque toinstantaneous load torque.

Every induction motor will have a maximum practicable operating fluxlevel (proportional to Volts/Hertz) inherent from its design. Themagnetic parts of the motor will saturate if operation is maintainedabove this maximum Volts/Hertz level producing extra losses andinefficient operation. In an A.C. traction drive, where speed is changedby changing the applied frequency of the motor voltage, the voltageapplied to the motor is increased linearly with speed to maintain thismaximum flux level up to the maximum voltage level of the system. Thispoint is the lowest speed (frequency) where maximum voltage is appliedand is called the "Voltage Corner Point" or VCP. Above this speed,voltage is held constant to maximum speed and flux decreases withincreased speed as explained above.

It is desirable in traction vehicles to transmit to the driving wheelsthe full horsepower (HP) capability of the prime mover up to thevehicle's maximum speed ("100% HP utilization"). When the traction motoris operated as a generator ("dynamic retarding") the HP capability isnot limited by the prime mover but only by the capability of theelectric transmission. It is desirable to have high retarding torquecapability at high speed. Since the induction motor's capability for agiven voltage is most limited at its highest operating speed(frequency), its retarding torque capability at maximum speed definesthe maximum retarding power level of the electric transmission systemwhich can be maintained down to the VCP speed with no increase in motorcurrent. A measure of the electric transmission system's capability at agiven HP level and inverter volt-ampere capacity is termed "wrap around"and is the ratio of the rated maximum speed divided by the speed atmaximum rated torque. A measure of value is the wrap around divided bythe cost (weight, size, etc.) of the transmission system.

Development of high torque also implies relatively high current whichrequires a relatively large inverter. High current also increasesheating in the motor which can result in damage or shut-down of themotor if equipped with thermal sensors. While such high current is ofconcern during motoring, electrical retarding of the vehicle, in whichthe motor acts as a generator, may require a voltage and current whichexceeds the capability of the inverter to obtain a desired power level.For example, constant braking torque at a high speed may require avoltage that is more than the maximum applied voltage during propulsionof the vehicle. Accordingly, it is desirable to provide a motor systemwhich can accommodate operation at high torque levels.

SUMMARY OF THE INVENTION

The present invention encompasses three methods which can be usedseparately or combined to increase the value of the A.C. electrictransmission system by increasing wrap around with no increase in costor, conversely, allowing smaller size equipment to obtain the same wraparound capability as larger equipment could without the inventions. Onemethod (motor transition) doubles the wrap around by halving the VCPspeed in the low speed region of motor operation. By moving the VCP tohalf the speed it is in the high speed connection maximum torque isobtained with one half the current level as would be needed if notransition was made since the voltage is doubled at the same speed. Asecond method (A.C. resistor insertion) is used in generating to gethigher torque capability at high speeds. By raising the voltage level atthe motor by the voltage drop across the resistor, the torque capabilityof the motor is increased by the square of the voltage increase withoutincreasing the voltage level impressed on the inverter. A third method(end ring air deflector) increases the continuous operating wrap aroundby raising the continuous torque rating of the motor through moreeffective cooling.

In one embodiment, an AC motor system includes an AC motor having arotor and a stator with the rotor having a plurality of air passagesextending axially for passing cooling air from one, end to another endof the rotor. The stator has end windings extending from slots passingaxially through the stator with the endwindings extending beyond theaxial ends of the rotor and stator. An air deflector is coupled to oneend of the rotor generally overlaying the air passages exiting from therotor. The air deflector has a generally curved configuration on asurface facing the air passages so that cooling air exiting the passagesfrom the rotor is directed radially upward and on to the statorendwindings. In one form, the air deflector may be formed as an end ringcoupled to rotor bars extending through the rotor. In another form, theair deflector may be attached to an end ring in such a position as todirect air radially upward between the rotor bars and on to theendwindings of the stator. In yet another form, the air deflector may beattached to the rotor shaft or formed integrally with a rotor end plate.

The endwindings exiting the stator are turns of a plurality ofelectrical conductors forming stator winding circuits. The statorwinding circuits are preferably wound so as to form a plurality of phasewindings. The phase windings may be coupled together by controllableswitching means to allow the windings to be selectively coupled into atleast two different electrical configurations. One of the electricalconfigurations may couple selected ones of the phase windings into aseries electrical circuit while another of the configurations mayarrange selected ones of the phase windings into a parallel electricalcircuit. In either of these series or parallel electrical circuit, themotor still includes the same number of electrical phases. The switchingmeans for selectively coupling the motor windings into the parallel orseries circuits is desirably responsive to motor speed. In one form, themotor may be coupled for driving a traction vehicle and the switchingmeans adapted for coupling the stator windings into a parallel circuitconfiguration during electrical retarding of the traction vehicle and inhigh speed motoring operations. The series connected configuration ofthe stator windings is used for low speed propulsion of the vehicle whenhigh torque is required. The switching means may be implemented aseither a contactor or a plurality of controlled semiconductor devices.

In another form of the invention, resistors are coupled in a serieselectrical circuit with each of the electrical phases of the statorwindings. Each of the resistors is further coupled in circuit with acontrollable switch for selectively bypassing phase current around anassociated one of the resistors. The controllable switches are openedwhenever the electrical motor is used in an electric braking modethereby forcing regenerative current to pass through the resistors. Inso doing, the voltage at the motors may be raised to a level higher thanthe normal limits dictated by the capability of the power sourceconnected to the motor. This will raise the motor torque handlingcapability to a higher level without having to increase the voltagecapability of the power source. During motoring, the resistors arebypassed by the controllable switches so that current from the powersource is not expended in the resistors.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, reference may behad to the following detailed description taken in conjunction with theaccompanying drawings in which:

FIG. 1 is a simplified circuit diagram of an AC motor system inaccordance with one form of the present invention;

FIG. 2 is a graph of performance characteristics for an exemplary ACmotor utilizing the switching between the series and parallel statorwinding connections illustrated in FIG. 1;

FIGS. 3 and 4 are graphs of motor operation in a retarding modeutilizing switching between the series and parallel stator windingconfigurations;

FIG. 5 and 5a illustrates conventional cooling air flow through an ACmotor;

FIG. 5A is a partial sectional view taken along lines 5A--5A of FIG. 5;

FIG. 6 illustrates an AC motor incorporating an annular air deflector inaccordance with one form of the present invention;

FIGS. 7A and 7B are side and end views of a motor incorporating anotherform of annular air deflectors;

FIG. 8 is a graph comparing operation of an exemplary AC motor with andwithout series resistance between the motor and an inverter.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified circuit diagram of an AC motor system inaccordance with one form of the present invention. An AC motor 10includes a plurality of stator windings formed in a conventional mannerto establish a three-phase stator winding configuration. Although threephases are illustrated as typical, other numbers of phases may beemployed in a motor using the invention. Each of the phase windings arepreferably divided into two equal segments, the phase A segments beingindicated as A1, A2, the phase B segments being indicated as B1, B2 andthe phase C segments being indicated as C1, C2. It will be appreciatedthat each segment may comprise multiple windings connected inparallel/series combinations and that each phase winding A, B and Ccould be separated into more than two segments.

The motor 10 is so constructed that each end of each segment A1, B1 andC1 is brought out to a respective one of a pair of terminals on acorresponding one of the switch means 12, 14 and 16. Additionally, oneend of each segment A2, B2 and C2 is brought out to another terminal ofa respective one of the switch means 12, 14 and 16. Other switch means18 and 20 are coupled between one end of each segment A1, B1 and C1. Byreference to FIG. 1, it can be seen that with the switches 12, 14, 16,18 and 20 in the condition indicated by the solid lines, the segments A1and A2 are connected in series circuit configuration as are the segmentsB1 and B2 and the segments C1 and C2. However, if the switches 12, 14,16, 18 and 20 are actuated such that the connections indicated by thedotted lines are made, the winding segment A2 is paralleled with segmentA1, segment B2 is paralleled with segment B1 and segment C2 isparalleled with segment C1. For convenience, the winding arrangement inwhich the phase windings are serially connected, i.e., the arrangementindicated by the solid line switch position, is hereinafter referred toas a series circuit configuration. Conversely, the arrangement developedwith the switches in the dotted line configuration is referred to as aparallel circuit configuration. While switches 12, 14, 16, 18 and 20have been indicated as separate devices, it will be appreciated thatthese switches may comprise independent contact sets on a single switch.Further, it is also contemplated that the mechanical switches could bereplaced with solid state components such as gate turn-off thyristors.

Referring to FIG. 2, there is shown a graph of performancecharacteristics for an exemplary AC motor incorporating the switchableseries circuit/parallel circuit invention of FIG. 1. It should be notedat this point that the graphs of motor operation in FIG. 2 and in FIGS.3, 4 and 8 are being provided only for illustrating the invention andrepresent characteristics of selected motors such as the GEBZ-6600series of AC electric traction motors. Values assigned to these graphswill vary as a function of motor characteristics. The curve labeled Trepresents motor torque over the range from zero to 4000 RPM. The motorreaches a preselected maximum horsepower (HP) at about 300 RPM and thenholds HP constant out to the maximum speed of 4000 RPM. The curvelabeled V represents applied motor voltage with the maximum value forthe illustrative motor system being set at 1400 volts. In the serieswinding configuration, the motor voltage corner point VCP₁ occurs atabout 740 RPM. Below this speed, voltage is reduced in order to avoidsaturation of the motor. The curve labeled I₁ represents current in theseries circuit configuration and can be seen to increase sharply belowthe motor voltage corner point VCP₁ with constant HP operation. At aspeed of about 1600 RPM, the torque margin, i.e,, the ratio of breakdowntorque (BDT) to load torque, has dropped to about 1.2 which representsless than a desirable margin of about 30%. An improved margin can beobtained by switching to the parallel circuit configuration at about1500 RPM which effectively doubles the voltage on each winding segmentand increases the available breakdown torque by a factor of 4. Curve I₂illustrates motor current in the parallel circuit configuration, whichcurrent is relatively constant from about 1500 RPM to the maximum speedof 4000 RPM. The voltage corner point indicated at 22 is the cornerpoint for the parallel circuit configuration and occurs at about 1400RPM. It is not desirable to hold the parallel circuit configurationbelow 1400 RPM since the current must increase linearly as voltagedecreases whereas the series circuit configuration would have relativelyconstant current in order to hold constant HP.

The exemplary curves of FIG. 2 illustrate the advantages ofreconfiguring the motor 10 into a parallel circuit configuration atspeeds above about 1500 RPM. Without such reconfiguration, it would benecessary to reduce motor HP in order to maintain an adequate torquemargin. The illustrative curves are for a type GEBZ-6600 traction motoravailable from General Electric Company. It will be appreciated that theparticular values of torque, current, voltage and speed will vary as afunction of the type of motor in which the invention is implemented.

Turning to FIGS. 3 and 4, there are shown graphs of motor operation in aretarding mode, i.e., when the motor is operated as a generatorconverting kinetic energy into electrical power. FIG. 3 shows voltage V,torque T and current I₂ for the motor 10 connected in the parallelcircuit configuration whereas FIG. 4 shows voltage V, torque T andcurrent I₁ for motor 10 connected in the series circuit configuration.The parallel circuit configuration, FIG. 3, provides significantadvantages in the electrical retarding mode. Most importantly, retardingtorque equivalent to 2760 HP can be developed at maximum RPM (4000) witha DC link voltage V of 1500 volts without exceeding a preselectedcurrent limit. However, torque T is limited to about 20,000 ft-lbswithout increasing current I, as indicated at T_(L). At a speed of aboveabout 3400 RPM, three is a current reduction and a correspondinghorsepower drop. Of course, these values are characteristic of aparticular motor and will vary with changes in motor design. The seriescircuit configuration, FIG. 4, is only useful for speeds below about1500 RPM. Above 1500 RPM, the torque margin becomes less than the 1.3limit discussed above. However, retarding torque can be developed athigher speeds in the series circuit configuration if the motor terminalvoltage is allowed to increase. One method for allowing increased motorterminal voltage without a concomitant increase in inverter size is toutilize the series connected resistors 34, 36 and 38, shown in FIG. 1,to drop part of the regenerated motor voltage.

Referring again to FIG. 1, each phase A, B and C of motor 10 is coupledto a respective phase link 24, 26 and 28 with each phase link beingconnected to a corresponding terminal of an inverter 30. The inverter 30is a conventional type utilizing controlled semiconductor devices suchas gate-turnoff thyristors for developing a variable frequency, variablepower output or for converting such variable frequency power to DC powerat DC link 32. The voltage output of the inverter 30 has a peak valuedetermined by the value of the voltage at the DC link 32. The effectivevalue of the voltage at the motor 10 may be varied by operating theinverter 30 in a pulse width modulation (PWM) mode wherein motorinductance is effective to smooth the current. The motor 10 is typicallycoupled for driving one or more wheels of a traction vehicle such as anoff-highway vehicle or a locomotive. An illustration of a systemincluding controls for such a vehicle is shown in U.S. Pat. No.5,070,959, the disclosure of which is hereby incorporated by reference.

Each of the AC phase links 24, 26 and 28 includes a series resistancemeans, illustrated as resistors 34, 36 and 38 and corresponding parallelconnected switches 40, 42 and 44. Although illustrated as mechanicalswitch means, the switches 40, 42 and 44 may be implemented insemiconductor form using controlled thyristors. An advantage of usingcontrolled thyristors for bypassing resistors 34, 36, 38 is that thefiring angle can be adjusted so as to control the effective resistancein each phase link. One pair of thyristors is shown in phantom at 45 andcould be used to replace switch 44. Similar arrangements of thyristorscould replace switches 40 and 42.

In the aforedescribed motor 10, the connection of the motor into theparallel circuit configuration allows the motor to produce higher fluxin electrical retard mode with the same amount of terminal voltage.However, in this configuration, the motor inductance is significantlyreduced, e.g., to about 25% of its value in the series circuitconfiguration. This reduction in inductance produces high current peaks,especially during PWM operation. The increase in peak current handlingrequirements for the semiconductor switches in the inverter requireslarger semiconductor sizes and greater cost. In addition, ripplecurrents are produced which may become as large as the fundamentalcurrent in the illustrative example. The resistors 34, 36 and 38 serveto reduce both ripple current and peak current to the inverter. Further,the resistors also serve to limit any fault current. Another advantageis that the regenerative voltage at the motor 10 may be allowed to gohigher than the DC link voltage to the inverter 30 due to the voltagedropped across the resistors. Consequently, the same torque can beproduced at a lower current with a higher motor terminal voltage andtorque capability of the motor at a given speed is increased.

In operation, the switches 40, 42 and 44 are preferably closed duringoperation of the motor in a propulsion mode so that all inverter poweris supplied to the motor 10. During electrical retarding, the switchesare opened so that current passes through the resistors. Depending uponthe desired retard effort and any desirable balance between dynamic andregenerative braking, the switches 40, 42 and 44 may be operated so asto close at some preselected low vehicle speed (motor RPM) , e.g., atabout 500 RPM. Alternately, the switches may be operated in response tosome preselected magnitude of motor current. Further, by usingcontrolled thyristors, the firing angle can be adjusted to vary theeffective resistance and to regulate the magnitude of retarding effort.Apparatus and methods for determining motor speed and/or current arewell known and specific disclosure of such is not believed necessary toan understanding of the invention. However, disclosure of both currentand speed monitoring is shown in the aforementioned U.S. Pat. No.51070,959.

While the use of resistors 34, 36 and 38 has been described inconjunction with the two different motor circuit configurations, it isalso proposed to use the resistors during electrical retard withoutswitching between motor circuits. In particular, if the motor 10 is onlyused in one configuration, e.g., a series circuit configuration, theresistors 34, 36 and 38 will allow the motor to be operated with ahigher terminal voltage, dropping some voltage across the resistors, anda corresponding lower motor current so that a higher retarding torquecan be developed without encroaching on the desired torque margin.

FIG. 8 is a graph of voltage current and torque comparing the operationof motor 10 with and without resistors 34, 36 and 38 when motor 10 isoperating in a retarding (regenerative) mode. With the resistors incircuit between the inverter 30 and the motor, the motor terminalvoltage can be increased up to about 900 volt even though the linkvoltage at the inverter is limited to about 630 volts from line toneutral. The graph at V1 plots motor terminal voltage as a function ofspeed. I₁ indicates motor current and T₁ indicates motor torque for thecase in which the resistors are connected in series with the motor.Without the resistors, motor voltage is limited to 630 volts,line-to-neutral, as shown at V₂. The resulting torque curve, orretarding effort, is indicated at T₂. At a speed of about 34 MPH for theillustrative example, the torque developed without the resistors isabout 1/2 that with the resistors, i.e., the effective retarding effortis ##EQU1## times the torque with the resistors.

In using the motor 10 or similar motors in high current modes, e.g., ina high speed electrical retard mode or a low speed, high torque mode,significant heat build-up occurs in the motor windings. Generally, someform of forced air cooling is used to preventing overheating of themotor. Referring to FIGS. 5 and 5A, the illustrated simplifiedcross-sectional views show a prior art motor 46 including an outerhousing 48, a rotor 50 and a stator 52. Only the upper half of the motor46 is shown since the motor is generally symmetrical about rotor axis54. The motor 46 is of the squirrel-cage type having a plurality ofcircumferentially spaced rotor bars 56. At each end of the rotor 50, thebars 56 are electrically and mechanically connected by end rings 58 and60. The end rings 58 and 60 are spaced from the respective ends of therotor 50 such that a portion of each rotor bar 56 extends beyond therotor ends.

The stator 52 includes a plurality of electrical conductors 62 arrangedin conventional phase windings and having end turns or endwindings 64and 66. At one end of housing 48 there is an air inlet 68 for receivingcooling air indicated by arrows 70. At an opposite end of housing 48 isan air outlet 72, which outlet may comprise a plurality ofcircumferentially spaced holes through housing 48. Cooling air 70 entersinlet 68, flows through passages 74 in rotor 50 and passages 76 instator 52, exiting through outlet 72. As indicated by arrows 70, thecooling air can bypass end turns 64 resulting in hot spots that can leadto motor failure.

FIG. 6 shows an improvement to motor 46 which assures that at least someof the cooling air 70 passes over or through end turns 64. Inparticular, an annular air deflector 78 is formed at the end of rotor 50adjacent end turns 64. The air deflector 78 may comprise the end ring58, reshaped to have a generally curved surface 80 facing the adjacentend of rotor 50. The curved surface 80 preferably has a generallyhyperbolic shape for redirecting air exiting the passages 74 into agenerally radially directed flow. Some of the redirecting air passesthrough the end turns 64 while the remainder passes over and around theend turns, thus assuring that the cooling air contacts the end turns 64.While the air deflector 78 is shown as being a modified end ring, itwill be recognized that the deflector may be formed as a separateelement attached to end ring 58. The deflector 78 in this latter formmay be formed from a lighter weight material such as a plastic.Furthermore, a deflector in this form could be created in multiplesegments for ease of assembly and merely bolted to the end ring 58.Further, the air deflector could be attached to the rotor shaft or couldbe integral with the rotor end plate.

FIGS. 7A and 7B are side and end views, respectively, of a motor 46having rotor end plates 82 and 84 used in implementing another form ofthe invention. The end plate 84 is a conventional structure having holeswhich align with passages 74 to permit air flow therethrough. The endplate 82 is modified to have a curved surface 86 facing the air exit endof at least some of the passages 74 so that at least some of the air isdirected radially outward onto end winding turns 64. The shape of thecurved surface 86 is generally the same as that described in FIG. 6. Oneadditional feature is to form the end plate 82 such that the curvedsurface 86 only intercepts air from every other one of the passages 74.For the remaining passages 74, corresponding holes are located in theplate 82 so that some of the air blows axially forward of the plate 82and is directed out through apertures 72 by end walls of the housing 48.It will be apparent that the same structure could be used with thepreviously described air deflectors 78, i.e., holes could be formedthrough the deflectors so that some air passes axially forward of therotor to the motor housing and wall.

While the invention has been described in what is presently consideredto be a preferred embodiment, many variations and modifications willbecome apparent to those skilled in the art. Accordingly, it is intendedthat the invention not be limited to the specific illustrativeembodiment but be interpreted within the full spirit and scope of theappended claims.

What is claimed is:
 1. An alternating current (AC) motor systemcomprising:an AC induction motor having a rotor and a stator, the rotorincluding a rotor winding comprising rotor bars passing through a coreand having bar extensions extending beyond each end of the core, ends ofthe rotor bar extensions being connected together by end rings, therotor bar extensions and end rings comprising end turns of the rotorwinding, the rotor still further including a plurality of radial airpassages formed between the rotor bar extensions, the end rings and theends of the core for passing cooling air from one end to another end ofthe rotor, the stator having end windings extending from each endthereof; and air deflector means coupled to said another end of saidrotor generally overlaying said air passages exiting from said anotherend, said deflector means having a generally curved configuration on asurface facing at least some of said air passages for directing coolingair exiting said passages into a generally radial flow direction andonto and through said end turns of said rotor and then onto said statorend windings adjacent said another end of said rotor.
 2. The system ofclaim 1 wherein said air deflector means comprises an annular memberhaving an outer diameter substantially corresponding to an outerdiameter of said rotor and having an inner diameter leas than a diameterof a circle inscribed on said another end of said rotor and passingthrough said air passages.
 3. The system of claim 1 wherein said statorand said rotor are mounted within a housing, said housing having an airinlet adjacent said one end of said rotor and having an air outletadjacent said another end of said rotor.
 4. The system of claim 3wherein said air outlet includes a plurality of circumferentially spacedopenings in a radially outer portion of said housing generally adjacentsaid stator end windings.
 5. The system of claim 1 wherein said curvedconfiguration of said air deflector means comprises a generallyhyperbolic curve.
 6. The system of claim 1 wherein said end windingscomprise turns of a plurality of electrical conductors forming statorwinding circuits, said circuits being wound to form a plurality of phasewindings, the system including controllable switching means forselectively coupling said phase windings into at least two differentelectrical configurations, one of said configurations comprising arelatively low speed, relatively high inductance configuration and theother one of said configurations comprising a relatively high speed,relatively low inductance configuration, said motor being operable ineither of said configurations during steady-state operation.
 7. Thesystem of claim 6 wherein one of said configurations connects selectedones of said phase windings in a series electrical circuit and anotherof said configurations connects selected ones of said phase windings ina parallel electrical circuit, each of said series and parallelelectrical circuits having the same number of electrical phases.
 8. Thesystem of claim 7 and including means responsive to motor speed forselectively switching said motor windings between said series andparallel electrical circuit configurations.
 9. The system of claim 8wherein said motor is coupled for driving a traction vehicle andincluding means for energizing said switching means for connecting saidmotor windings into said parallel circuit configuration duringelectrical retarding of the vehicle.
 10. The system of claim 9 whereinsaid motor is connected in said series circuit configuration during lowspeed propulsion of said vehicle.
 11. The system of claim 10 whereinsaid switching means comprises a plurality of contactors.
 12. The systemof claim 7 and including resistor means coupled in series electricalcircuit with each of said electrical phases, each of said resistor meansincluding a second controllable switching means coupled in paralleltherewith for selectively bypassing phase current around said resistormeans.
 13. The system of claim 12 wherein said second controllableswitching means is operable to bypass current around said resistor meanswhen said motor is operated in a propulsion mode.
 14. The system ofclaim 13 wherein said second controllable switching means is operable todirect current through said resistor means when said motor is operatingin a relatively high speed electrical braking mode.
 15. The system ofclaim 12 wherein said switching means comprises a plurality ofsemiconductor switching devices, selectively gated into conduction in amanner to vary the effective value of said resistor means.
 16. Analternating current (AC) motor system comprising:an AC motor having arotor and a stator, said stator having a plurality of electricalconductors forming a preselected number of electrical winding circuits;first controllable electrical switching means coupled in circuit withsaid winding circuits, said switching means being operable toselectively couple said winding circuits into at least two differentelectrical configurations, each of said configurations having the samenumber of electrical phases; means responsive to speed of said motor forselectively switching said windings between said configurations, one ofsaid configurations comprising a relatively low speed, relatively highinductance configuration and the other one of said configurationscomprising a relatively high speed, relatively low inductanceconfiguration, said motor being operable in either of saidconfigurations during steady-state operation; resistor means coupled inseries electrical circuit with each of said electrical phases; secondcontrollable switching means coupled in parallel electrical circuit withsaid resistor means for selectively bypassing current about saidresistor means; and means responsive to operation of said motor in aregenerative electrical mode for energizing said second switching meansfor forcing current through said resistor means.
 17. The system of claim16 wherein said motor is coupled for selectively propelling andelectrically retarding a traction vehicle, said resistor means beingcoupled in a series current path with said electrical phases during atleast high speed electrical retarding of said vehicle.
 18. The system ofclaim 17 wherein said two different configurations comprise a firstconfiguration having selected windings connected in series circuit ineach of said phases and a second configuration having said selectedwindings connected in parallel circuit in each of said phases.
 19. Thesystem of claim 18 wherein said windings are connected in said firstconfiguration for high torque, low speed propulsion of said vehicle. 20.An alternating current (AC) motor system comprising:an AC inductionmotor having a rotor and a stator, the rotor including a rotor windingcomprising rotor bars passing through a core and having bar extensionsextending beyond each end of the core, ends of the rotor bar extensionsbeing connected together by end rings, the rotor bar extensions and endrings comprising end turns of the rotor winding, the rotor still furtherincluding a plurality of radial air passages formed between the rotorbar extensions, the end rings and the ends of the core for passingcooling air from one end to another end of the rotor, the stator havingend windings extending from each end thereof, said end windingscomprising end turns of a plurality of electrical conductors formingstator winding circuits arranged in a plurality of electrical phasewindings; air deflector means coupled to said another end of said rotorgenerally overlaying at least some of said air passages exiting fromsaid another end, said deflector means having a generally curvedconfiguration on a surface facing said at least some air passages fordirecting cooling air exiting said passages into a generally radial flowdirection and onto and through said end turns of said rotor and thenonto said stator end windings adjacent said another end of said rotor;and resistor means selectively coupled in series circuit with each ofsaid phase windings.
 21. The system of claim 20 and including a sourceof variable frequency, variable voltage power connected in circuit withsaid motor, and further including switching means coupled in circuitwith said resistor means for connecting said resistor means in seriescircuit between said motor and said power source.
 22. The system ofclaim 21 wherein said motor is coupled for selectively driving andretarding a traction vehicle, said switching means being operable forconnecting said resistor means into said series circuit duringrelatively high speed electrical retarding of the vehicle.
 23. Thesystem of claim 22 and including a variable frequency, variable voltagepower source connected in circuit with said motor, said resistor meansbeing selectively coupled in series circuit between said power sourceand said motor during electrical braking of said vehicle.
 24. The systemof claim 14 wherein said motor is coupled for selecting propelling andretarding a traction vehicle, said electrical braking mode correspondingto electrical retarding of said vehicle by said motor.
 25. The system ofclaim 24 and including a variable frequency, variable voltage powersource connected in circuit with said motor, said resistor means beingselectively coupled in circuit between said power source and said motorduring electrical braking of said vehicle.